In a mobile communication system, a communication terminal searches for a neighboring cell and measures the reception quality of a radio wave from the detected neighboring cell (hereinafter referred to as “quality measurement”) when there is a decline in the quality of communication with a cell it is currently connected to (hereinafter referred to as a “serving cell”). If a neighboring cell of better reception quality than the serving cell is found as a result, a network controller causes the communication terminal to perform a handover to the neighboring cell.
The search and quality measurement for a neighboring cell are major factors in terms of reducing the power consumption of a communication terminal. Basically, if a serving cell is of a sufficiently good quality, the communication terminal is presumed to have no need for performing the search and quality measurement for a neighboring cell, since a communication terminal only needs to be connected to a serving cell. A threshold value for determining whether or not to perform a neighboring cell search (this threshold value is called “S-measure” in LTE) is therefore specified (Non-patent document 3). This threshold value is herein called a “search threshold value.”
FIG. 24 illustrates a search threshold value. As shown in FIG. 24, when the measured value of the reception quality of a serving cell is above a search threshold value, a neighboring cell search is not performed since the quality is good and a handover is presumed to be unnecessary. On the other hand, when the measured value of the reception quality of a serving cell is below a search threshold value, a neighboring cell search is performed since the quality is bad and a handover may be made. Consequently, a neighboring cell search is performed only when required, and the power consumption of a communication terminal can be reduced.
By the way, LTE-advanced is now being standardized by 3GPP so as to be a candidate for a wireless communication system adopted into IMT-advanced. In this LTE-advanced standardization, carrier aggregation, in which a plurality of component carriers are simultaneously assigned to a communication terminal, is under review for improvement in the throughput of a communication terminal.
FIG. 25 is a conceptual diagram illustrating carrier aggregation. In the example shown in FIG. 25, there are component carriers f1 to f3 of a bandwidth of 20 MHz. A communication terminal supporting carrier aggregation (e.g. a Rel-10 communication terminal) uses the component carriers f1 to f3 simultaneously to communicate with a bandwidth of 60 MHz.
On the other hand, a communication terminal which dose not support carrier aggregation (e.g. a Rel-819 communication terminal) connects to one of the component carriers f1 to f3 to communicate over 20 MHz.
Keeping the bandwidth unchanged as above allows previously released communication terminals (e.g. Rel-8/9) to be supported as well, and can improve the throughput of communication terminals to be newly released (e.g. Rel-10 communication terminals). This is one merit of carrier aggregation.
Note here that a communication terminal incompatible with carrier aggregation regards each circle of the carriers f1 to f3 shown in FIG. 25 as a cell. The cell is defined by 3GPP (Non-patent document 1). Further efficiency is now under study in consideration of implementing carrier aggregation. Scenarios for enhancing efficiency will be described below.
(Scenario 1)
FIG. 26 shows one scenario for further enhancing the efficiency of carrier aggregation. A component carrier f1 includes a synchronization channel, broadcast information, an L1 control channel, and the like, and can alone provide services to a communication terminal. Component carriers f2 and f3 do not include a synchronization channel nor broadcast information, and a communication terminal cannot detect those component carriers alone. This is because a communication terminal detects a component carrier (which is called “cell detection” in Rel-8) by receiving a synchronization channel in a cell search process.
A communication terminal cannot be on standby (which is called “camp on”) nor establish a call on the component carriers f2 and f3. The standby and call establishment are allowed by receiving broadcast information (more specifically, Master Information Block (MIB), System Information Block 1 (SIB1), and System Information Block 2 (SIB2) in broadcast information) after cell detection. A communication terminal therefore cannot be on standby on the component carrier concerned unless there are both a synchronization channel and broadcast information.
In this scenario, a communication terminal in an idle state (RRC_IDLE) detects only the component carrier f1 and then begins to be on standby. After that, the communication terminal performs a call establishment process, comes into an active state (RRC_CONNECTED), and then adds the component carriers f2 and f3 in accordance with an instruction from the network side, to perform carrier aggregation. Since the communication terminal may require reception of broadcast information even after it comes into an active state, there may be an operation in which the communication terminal continues to use the component carrier f1 and uses the component carriers f2 and f3 just as additions. FIG. 27 shows one example of a process of adding the component carriers f2 and f3.
A communication terminal which does not support carrier aggregation (e.g. a Rel-8/9 communication terminal) will use only the component carrier f1 even after it comes into an active state.
(Scenario 2)
FIG. 28 shows another scenario for further enhancing the efficiency of carrier aggregation. A component carrier f1 includes a synchronization channel, broadcast information, an L1 control channel, and the like, and can alone provide services to a communication terminal. Component carriers f2 and f3 do not include an L1 control channel, and a communication terminal cannot detect those component carriers alone. This is because a communication terminal cannot determine which resource it should use when there is no L1 control channel, since it is notified of which resource it should use through an L1 control channel.
As with the previously described scenario, a communication terminal in an idle state cannot be on standby on the component carriers f2 and f3, and a communication terminal which does not support carrier aggregation (a Rel-8/9 communication terminal) also cannot use the component carriers f2 and f3.
In the above examples, the component carrier which can provide all services and to which a communication terminal must be connected at the very least (the component carrier f1 in FIGS. 26 and 28) is sometimes called a backward compatible component carrier. This is because it can sometimes support a communication terminal of Rel-8/9 and the like as well (Non-patent document 2). Conversely, component carriers other than the above are sometimes called non-backward compatible component carriers.
While the downlink and uplink are not particularly distinguished from each other in the above description, the description basically centers on downlink operation. The downlink and uplink correspond one-to-one with each other in LTE Rel-8.
FIG. 29 shows an “operation in LTE Rel-8.” That is, when a frequency 1 used for the downlink and a frequency 4 used for the uplink are paired with each other and a communication terminal uses the frequency 1 to receive, it uses the frequency 4 to transmit. Similarly, frequencies 2 and 5, as well as frequencies 3 and 6, are paired with each other. The process in FIG. 27 is thus shown only for the downlink for the purpose of simplification though different component carriers are actually used for reception and for transmission.
FIG. 30 shows an example of possible carrier aggregation in which the downlink and uplink are asymmetric. There may also be such an asymmetric operation in future extensions. However, the invention can be applied to either case where the uplink and downlink are symmetric or asymmetric. The description below will center on downlink component carriers.